Link found between neuron firing and behavior

(April 3, 2009)--UTSA researchers are one step closer to understanding the neurobiology that allows people to learn motivated behaviors by associating environmental cues with rewarding outcomes, according to a study just published in the Proceedings of the National Academy of Sciences online Early Edition.

"Our research findings provide a direct functional link between the bursting activity of midbrain dopamine neurons and behavior," said said Paladini, who also is a member of the UTSA Neurosciences Institute. "The research has significant applications for the improvement of health, because the dopamine neurons we are studying are the same neurons that become inactivated during Parkinson's Disease and with the consumption of psychostimulants such as cocaine and amphetamine."

Midbrain dopamine neurons fire in two characteristic modes, tonic and phasic, which are thought to modulate distinct aspects of behavior. When an unexpected reward is presented to an individual, midbrain dopamine neurons fire high-frequency bursts of electrical activity. Those bursts of activity allow us to learn to associate the reward with cues in our environment, which may predict similar rewards in the future.

A protein called the NMDA receptor, which is expressed on the surface of the dopamine cells, facilitates the burst of electrical spikes observed in dopamine neurons. In this study, researchers removed the NMDA receptor from the dopamine cells only, leaving the dopamine neurons unable to fire bursts. The cells would otherwise fire normally.

When researchers placed the mice in reward-based situations, they found that the mice without the NMDA receptor in their dopaminergic neurons could not learn tasks that required them to associate sensory cues with reward. Those same mice, however, were able to learn tasks that did not involve an association with rewards.

"Now that we know NMDA receptors are required for burst firing in dopamine neurons, we need to explore the mechanisms by which NMDA receptor-mediated bursting is regulated or gated," said Lobb, who is currently pursuing a Ph.D. in neuroscience at UTSA.